U.S. Pat. No. 6,308,531 discloses a process of the aforementioned type, in which a natural gas stream is liquefied by means of a first refrigeration cycle involving the condensation and vaporisation of a hydrocarbon mixture. The temperature of the gas obtained is approximately −100° C. Then, the LNG produced is sub-cooled to approximately −170° C. by means of a second refrigeration cycle known as a “reverse Brayton cycle” comprising a staged compressor and a gas expansion turbine. The refrigerating fluid used in this second cycle is nitrogen.
A process of this type is not completely satisfactory. The maximum yield of the cycle known as the reverse Brayton cycle is limited to approximately 40%.
An object of the invention is therefore to provide an autonomous process for sub-cooling an LNG stream, which has an improved yield and can easily be employed in units of various structures.